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Understand Incretins
Merck

Incretins play an active role in mediating pancreatic beta- and alpha-cell responses in healthy individuals

Incretins are naturally occurring hormones that the gut releases throughout the day; the level of active incretins increases significantly when food is ingested. 24,25

Endogenous incretins GLP-1 (glucagon-like peptide 1) and GIP (glucose-dependent insulinotropic peptide) facilitate the response of the pancreas and liver to glucose fluctuations through their action on pancreatic beta cells and alpha cells.

GIP and GLP-1 are the 2 major incretin hormones in humans: 13,18

  • GIP is a 42-amino-acid peptide derived from a larger protein (ProGIP) and is secreted by endocrine K cells mainly present in the proximal gastrointestinal (GI) tract (duodenum and proximal jejunum) 18,26
  • GLP-1 is a 30-amino-acid or 31-amino-acid peptide derived from a larger protein (proglucagon) and is secreted by L cells located predominantly in the distal GI tract (ileum and colon) 18,26

These incretins are released from the gut in response to ingestion of food 19 and collectively contribute to glucose control by:

  • Stimulating glucose-dependent insulin release from pancreatic beta cells (GLP-1 and GIP): 18
    • Insulin increases glucose uptake in peripheral tissues (mainly muscle and fat) and suppresses glucose production from the liver. 21,22
  • Decreasing glucagon production from pancreatic alpha cells (GLP-1) 22,23 when glucose levels are elevated.

The combination of increased insulin production and decreased glucagon secretion reduces hepatic glucose production when plasma glucose is elevated. 18

The physiologic activity of incretins is limited by the enzyme dipeptidyl peptidase-4 (DPP-4), which rapidly degrades active incretins after their release. 27,28

The incretin effect is diminished in type 2 diabetes
  • Levels of GLP-1 are decreased. 29
  • The insulinotropic response to GIP is diminished but not absent. 30
  • Defective GLP-1 release and diminished response to GIP may be important factors in glycemic dysregulation in type 2 diabetes. 13